Robert Hughes recalls Bacon saying in an interview:"Some people think that my paintings are horrible. Horrible! But then you only need to think about the meat on your plate."

Bacon has not only a horror of religion (never forget that he was born and raised in Ireland), but also an acute dislike of narrative painting. His purpose is not storytelling. It is sensation. "Some paint comes across directly on to the nervous system, other paint tells you the story in a long diatribe through the brain." That distinction, between "brain" and "nervous system", thinking and feeling, was always of capital importance to him. It lies at the root of his lifelong denial that he was in any way an "expressionist"; to him, his paintings were realistic, rather intensely so. Like certain still lives by Goya, or like the more recent and no less extraordinary masterpiece of a skinned rabbit by Antonio López García, they make you think about the meat on your plate.

....

...the famously filthy mess that was Bacon's studio at Reece Mews - the piles and sticky avalanches of photos, books, clipped newsprint, booze-stained scribbles on the verge of becoming drawings, squished paint tubes and every imaginable ingredient of clutter that covered the horizontal and vertical surfaces, tables and walls, like some illegible compost in which, like moulds or somewhat alien life-forms, his future pictures were brewing and his past ones decaying. This stuff was more than rubbish. It was an archive, admittedly a staggeringly disordered one. And fortunately, instead of throwing all this crap out, Bacon's heir, John Edwards, saw to it that it was all collected and given to the Hugh Lane Gallery in Dublin. At first one greeted this news with scepticism: wasn't it some kind of joke, the artist so famous that even his shit is adored having years of excreta preserved in one of the principal museums of his native country? But no. The rubbish was a precious archaeological record. This becomes clear when you read the Tate catalogue, whose authors delve often brilliantly and always inquisitively into its resources. Of these, the chief is photography.

There is scarcely an important western artist of the past hundred years around whom a book could not be spun, and a show constructed, with the title "Fred X and Photography". Perhaps one or two, such as Mondrian. Even Pierre Bonnard, once thought of as a rapturous apostle of the unmediated eye, used snapshots. But Bacon's resort to photography, both still and cinematic, was constant, obsessive and over-the-top. Its sources and results have an enormous span, from the relatively familiar - Dr Goebbels orating, terrified crowds scattering from the tsar's police, or the bloody face of the nurse on Eisenstein's Odessa Steps, peering hysterically through her broken spectacles - to the utterly obscure. There are bits of Picture Post and images from those resources of gay porn, the body-culture magazines of the 50s. Sometimes the obscure details lie within images themselves famous. For instance, there is a well-known photograph of a racing-car at Le Mans, in which the speed of the machine and the panning camera movement turn the wheels into forward-leaning ellipses, distorted cartoonwise. So striking is this effect and so dominant the machine's image that few people so much as notice the figures in the background, on the verge of the track. But Bacon did, and he stole a pair of them, enlarging them for the right panel of Crucifixion (1965), where, in their odd soft hats, they look threateningly like a pair of Australian yobs leaning on a bar.

There are also photographs that Bacon's paintings made famous, but few except insiders knew about before Bacon swiped them. Obvious examples are the images from Eadweard Muybridge's Animal Locomotion of the 1880s, albums of sequential shots of human and animal movement - wrestlers, a trotting dog, or the haunting "paralytic child walking on all fours". Muybridge is such a classic figure in the history of photography now that it's hard to imagine a time when his work wasn't recognisable, but that was the time when Bacon came in. Analogous to these are some of the extraordinary images in KC Clark's Positioning in Radiography, a medical album published in 1939, in which the bodies of hospital patients are twisted and spraddled to display their tumours and other malformations to the searching gaze of the x-ray lens, calling to mind the words of Bacon's favourite modernist poet, TS Eliot: "The wounded surgeon plies the steel / That questions the distempered part". All this was grist to Bacon's mill. It enabled him to find new configurations in the body that were not the result of "artist's licence", not formal distortions gained from invention, but real, actual and corporeal. They had something to do with the traumatised flesh of accident victims, and something to do with the froggy postures of sex, but fully partook of the nature of neither.

And from the Independent, Michael Glover on the viscosity of Francis Bacon

"Fundamentally, man is alone with himself, and that self is not a pretty sight. We must never forget that there is a skull beneath the skin, and that the flesh – though capable of prettification – is, in the end, so much raw meat. What is more, we live in an ever-shifting world, a world of shifting values, a world – like Bacon's paintings – in which everything appears to be unstable, even the flesh that slathers across the face of the bony human skull.

It is the exact same vision that Sartre the novelist gives us through the character of Antoine Roquentin, the autodidact hero of La nausée (Bacon too was an autodidact). The world is viscous, disgustingly treacly in its perpetual instability, forever slipping away from us. The self, too, is disgustingly unstable, perpetually posed to be refashioned by a wholly unpredictable future.

Bacon's vision, as he summarised it in 1964, almost 30 years after the publication of Sartre's novel, is at one with this. "I would like my pictures to look as if a human being had passed between them, like a snail, leaving a trail of the human presence and memory trace of past events as the snail leaves its slime," he said. Yuck."

Francis Bacon opens at Tate Britain on 11 September, and runs to 4 January 2009